FROM TWELVE TO THIRTY-SIX SOMITES 193 
homologized with these. On the other hand, they appear at the 
same time as the first glomeruli of the mesonephros (q. v.) and 
possess, by way of the intermediate tubules, undeniable resem- 
blance to the latter. 
At the stage of 10 somites the pronephros is represented by a series 
of thickenings of the somatic layer of the intermediate cell-mass extend- 
ing from the fifth somite backward to the segmental plate. In an embryo 
of 13 somites the connection between the somite and nephrotome is lost, 
and the pronephric tubules from the ninth to the thirteenth somites 
have united to form the beginning of the Wolffian duct. 
In an embryo of 16 somites a single pronephric tubule was found 
at the level of the hind end of the fifth somite, and was very distinct 
on one side but hardly discernible on the other. Its posterior continua- 
tion was soon lost, and the next distinct tubules were between the ninth 
and tenth somites; from here back there was a tubule opposite the hind 
end of each somite to the fifteenth, which was the last, and the duct was 
continuous. 
In an embryo of 21 somites, one finds only isolated remnants of the 
pronephros in front of the eleventh somite; from here to the fifteenth 
the tubules are well developed and retain their connection both with 
the Wolffian duct and the lateral plate. The Wolffian duct extends 
back of this place to the region of the posterior half of the segmental 
plate. 
At the 35s stage the pronephric tubules are much degenerated, 
but the nephrostomes usually remain. In one embryo there was found a 
well-developed pronephric tubule on each side in the thirteenth somite. 
That of the left side had a wide nephrostome, the lumen of which stopped 
short of the tubul ; the nephrostome of the right side was rudimentary. 
On the right side the Wolffian duct extended no farther forward, but 
on the left side it was continued to the eleventh somite, and rudimentary 
pronephric strands uniting it to the ccelomic epithelium existed in both 
eleventh and twelfth somites. Here the Wolffian duct stopped. But 
isolated pronephric rudiments and minute nephrostomes were found on 
both sides as far forward as the tenth somite. 
The Wolffian Duct. The Wolffian duct consists according to 
the foregoing account of two parts, (1) an anterior division formed 
by the union of the pronephric tubules, and (2) a posterior divi- 
sion that arises as an outgrowth of the anterior part. The latter 
grows backward above the intermediate cell-mass as a_ solid 
cord (Fig. 107), apparently by active multiplication of its own 
cells, without participation of the neighboring mesoderm or 
